A jack-up rig is an offshore oil and gas exploration drilling structure or a work-over platform being used in shallow water, typically in water with depths up to 500 feet. The jack-up rig usually comprises a floatable hull with a deck or working platform, and three or four legs, where the legs provide support for the floatable hull in elevated conditions. After the jack-up rig arrives at a location, its legs are lowered until the legs touch the underneath seabed and rest on the soil on the seabed. Then, the hull of the jack-up rig may be jacked up using a jacking system to raise the working platform of the jack-up rig above the water so that the jack-up rig is safe to be operated in open water situations where water movement is experienced.
The legs of a jack-up rig are commonly tubular columns or trusses, each truss comprising vertical chords connected with cross braces that are normally diagonally disposed. The legs normally terminate in a jack-up foundation that rests on the seabed. The foundation provides an enlarged bearing area so as to provide an adequate bearing capacity and reduce the pressure exerted on the soil of the seabed. Resultantly, this reduces the penetration depth of the legs that is required by the foundation to support the jack-up rig, allowing the jack-up rig to be operated in a greater variety of locations and soil types with the available leg length.
Modern jack-up rigs are typically equipped with individual spudcan footings which are connected to each leg of the jack-up rig. This allows the jack-up rig to be supported on uneven seabeds or slopes or in the cases whereby the elevation of each leg is needed to be independently adjusted relative to the other legs. A spudcan is typically shaped like a top, having a generally conical upper half connected to the leg and a generally conical lower half or base for contact with the seabed. The conical base helps ensure some penetration into the seabed, even in hard soils, so as to provide some anchoring of the legs into the seabed.
Since the spudcan footing may rest with a shallow penetration, which typically occurs in sandy or very stiff seabed, it is common that the resulting horizontal bearing capacity of the jack-up rig is relatively low compared to the anticipated horizontal load. In such situations, skirted or caisson foundations are typically adopted to provide a larger embedment effect of the footing into the founding stratum. The skirt or caisson effectively forms a vertical ring or tube that walls off the foundation from the surrounding seabed. If the self weight is insufficient to allow full penetration of the skirt, suction is typically applied within the internal skirt to create under-pressure that results in further penetration of the skirt until reaching the design depth.
In most cases, the overall behavior of a skirted foundation can often be justifiably treated as an embedded solid foundation block with its base resting on the same depth as the skirt tip. The skirt not only improves the horizontal capacity but also increases combined bearing capacity of the shallow foundations by transferring the bearing load to a larger depth through soil mass confined within the skirt. The skirt is generally able to increase the bearing capacity in vertical, horizontal, and moment directions.
The full advantages of skirted or caisson foundations, however, may be hindered if soft material of significant thickness is present above the founding stratum. Although the very soft material may be thick and the resulting penetration of the spudcan footing is relatively deep, the horizontal capacity is still governed mostly by the effective spudcan base embedment into the founding stratum. After the foundation installation, the soft material trapped within the skirt, sandwiched between the foundation base and the founding stratum, may consolidate over time under operational load or result in significant additional settlements when subjected to design load. In addition, the skirt penetration may be limited by the trapped soft material which in turn results in significantly less skirt penetration into the founding stratum than designed for.
In order to ensure the effectiveness of the skirted foundation in such a soil condition, removal of the trapped soft material from the caisson interior is therefore critical, and challenging particularly for offshore applications. Certain suction pumps or eductor systems may be able to suck out soft material (of up to particular shear strengths and densities) from the caisson interior. However, it is difficult to ensure effective removal of the soft material by such a means particularly when it encompasses a large area underneath a foundation and involves a massive volume of soil.
One alternative for eliminating the soft material is ground preparation work such as excavation prior to foundation installation. However, under-water excavation in soft seabed is often problematic, if not impractical, particularly when the soft material is significantly thick. Additional means such as sheet piles installation may be required to prevent the surrounding soil from collapsing back to the excavated area. On the other hand, soil improvement technique might not be feasible or good enough to provide sufficiently stiff treated soil. Potential environmental pollution may also be of concern with applications of such methods.
In such a soil condition, piled foundation or its combination with shallow foundation systems can be designed to provide required bearing capacity. However, when a foundation system is required to be mobile, such as in the ease of jack-up rigs, use of piles or its combination therewith is not preferred since removal of the piled foundation is not practical.